Impact detectors are well known. They are generally employed in commercial and industrial applications to provide some indication (change of resistance, switch closure, output voltage or current) of the presence or passage of a person or vehicle to, for example, open a door or gate, sound an alarm, etc. They are also commonly employed in residential, commercial and industrial applications for intrusion detection.
Piezoelectric films ("piezo films") are well known to have application to impact detection. One significant benefit of piezo film is its dynamic property, i.e., it spontaneously generates a voltage (charge) pulse or transient upon impact, and therefore is useful as an impact detector that does not require voltage input or elaborate interface circuitry. Moreover, piezo films are inexpensive and impact detectors that employ piezo film are relatively simple.
One area where impact detectors have found use is in outdoor applications. For example, impact detectors are commonly embedded in roadways, e.g., near intersections, to signal the presence of a vehicle to a so-called "actuated" (as opposed to "pre-timed") traffic signal controller. Other potential outdoor applications for impact detectors include embedded mats in a parking lot for vehicle detection or in a yard area for intrusion detection.
It would be desirable to employ piezo film mats for these outdoor applications, but it is well known that piezo film exhibits another phenomenon that, heretofore, has rendered it unsuitable for outdoor impact detection applications. Piezo film also produces a voltage (charge) output when the temperature of the film (due to, e.g., sun light or environmental temperature) changes rapidly. The output from the film due to pyro effects is often indistinguishable from the output due to impact. For example, FIG. 1 shows piezo film output due to sunlight (simulated by an array of 300 watt infrared lamps) and FIG. 2 shows a foot step output on a piezo film mat. Of course, if piezo film is to be used for outdoor applications, and particularly in areas exposed to sunlight, then elimination or minimization of the pyro effect is crucial.
When moderate changes in temperature occur, the output due to the pyro effect is predominantly low frequency. In this case, a high pass filter can be employed to eliminate the pyro effect. In many outdoor applications, however, the frequency range of the pyro effect output is higher or identical to that of the impact output, and therefore this method cannot be employed.
It is therefore desirable to provide means, such as, but not necessarily limited to, a piezo film structure, that eliminates or minimizes the pyro effect of piezo film so that piezo film mats may be employed in outdoor applications. Such means should be inexpensive to implement and simple, yet reliable. The present invention achieves these goals.